1. Field of the Invention
The present invention relates to an improvement in a valve-regulated lead-acid battery.
2. Description of the Related Art
In recent years, enhancing the specific power (W/kg) and specific energy (Wh/kg) of a valve-regulated lead-acid battery and improving the life performance thereof have been urgent subjects to be developed.
In order to enhance the specific power and specific energy of the valve-regulated lead-acid battery, it is necessary to improve the discharge characteristic of the battery and reduce the weight thereof. In order to improve the discharging characteristic of the valve-regulated lead-acid battery, it is known that it is efficient to thin the positive and negative plates used for the valve-regulated lead-acid battery as well as to increase the geometrical surface area thereof and to improve the utilization of positive and negative active materials. Further, in order to decrease the weight of the valve-regulated lead-acid battery, it is considered desirable to decrease the amount of the negative active material which has comparatively little effect on the discharge characteristic of the valve-regulated lead-acid battery and to reduce the weight of the portions which do not contribute to electric power generation in the elements of the valve-regulated lead-acid battery.
However, in the case of thinning the positive plate, the life performance of the valve-regulated lead-acid battery is attenuated by corrosion of a positive electric collector.
In addition, when the porosity of the negative and positive active materials is increased in order to improve the utilization of the positive and negative active materials, the life performance becomes poor because of the deterioration of the positive and negative active materials.
Further, when the amount of the negative active material in the valve-regulated lead-acid battery is decreased so that the theoretical capacity (Ah) of the negative active material becomes less than that of the positive active material, the depth of discharge of the negative active material during discharging of the valve-regulated lead-acid battery becomes deeper than that of the positive active material. As a result, lead sulfate is generated at a greater rate in the negative active material than in the positive active material. Further, because of the gas recombinant reaction which is the greatest feature of the valve-regulated lead-acid battery, oxygen gas generated from the positive plate during charging of the battery reaches the negative plate so that the reduction reaction of the oxygen gas in the negative plate occurs more preferentially than the charging reaction in the negative plate. Consequently, it is impossible to charge the negative active material by any charging.
For these two reasons, in the valve-regulated lead-acid battery in which the theoretical capacity of the negative active material is less than that of the positive active material, sulfate is likely to be created and accumulated in the negative active material during both discharging and charging. Therefore, the negative active material deteriorates during repetition of the charging/discharging, and the life characteristic of the valve-regulated lead-acid battery becomes poor.
In addition, if the weight of electric collectors for the positive and negative which do not contribute to electric power generation in elements of the valve-regulated lead-acid battery is light, the life characteristic becomes poor because of corrosion of the collector for the positive and the deterioration of the negative active material described as follows.
The negative plate of the valve-regulated lead-acid battery includes a collector and a negative active material which is a spongy metallic lead. Accordingly, the negative active material in a completely charged state has a conductivity so that it can be discharged even when the weight of the negative collector is very small. However, lead sulfate, which is a discharging product of the negative active material, has no conductivity. Consequently, in order to charge the negative active material after discharging, a charging potential has to be applied to the negative material through the negative electric collector. Here, when the weight of the negative electric collector is small, the current density of the negative electric collector is increased. Therefore, only the negative active material at the portion in the proximity of the electric collector can be charged. Namely, the phenomenon that lead sulfate is accumulated in the negative active material occurs. This phenomenon is remarkable as the negative active material is thinned and the utilization of the negative active material is improved.
Conventionally, an electrically conductive additive such as carbon has been added to the negative active material of the lead-acid battery, regardless of whether the battery is a valve-regulated or flooded type. This tends to improve the charging property when the negative active material is charged in an unformed state. The additive is commonly added to the negative active material in an amount of 0.2 weight %. This value of 0.2 weight % is sufficient to improve the charging property when the negative active material in an unformed state is charged. Further increasing the amount of these expensive additives leads to higher cost. The flooded type lead-acid battery has a drawback that the conductive additive solves out from the negative plate into an electrolyte to make the electrolyte dirty.
Even when the amount of the conductive additive such as carbon in the negative active material in the common lead-acid battery is exceeded, as disclosed in Unexamined Japanese Patent Publication Sho. 59-79973, there is no specific performance improvement other than that in the formation characteristic of the negative plate. This is one of the reasons that the amount of the conductive additive was limited to about 0.2 weight %.